A guest OS that supports memory hotplug / memory devices must during
boot be aware of the maximum possible physical memory address that it might
have to handle at a later stage during its runtime.

For example, the maximum possible memory address might be required to
prepare the kernel virtual address space accordingly (e.g., select page
table hierarchy depth).

On s390x there is currently no such mechanism that is compatible with
paravirtualized memory devices, because the whole SCLP interface was
designed around the idea of "storage increments" and "standby memory".
Paravirtualized memory devices we want to support, such as virtio-mem, have
no intersection with any of that, but could co-exist with them in the
future if ever needed.

In particular, a guest OS must never detect and use device memory
without the help of a proper device driver. Device memory must not be
exposed in any firmware-provided memory map (SCLP or diag260 on s390x).
For this reason, these memory devices will be places in memory *above*
the "maximum storage increment" exposed via SCLP.

Let's provide a new diag500 subcode to query the memory limit determined in
s390_memory_init().

Message-ID: <20241219144115.2820241-8-david@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
---
 hw/s390x/s390-hypercall.c | 12 +++++++++++-
 hw/s390x/s390-hypercall.h |  1 +
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/hw/s390x/s390-hypercall.c b/hw/s390x/s390-hypercall.c
index f816c2b1ef..ac1b08b2cd 100644
--- a/hw/s390x/s390-hypercall.c
+++ b/hw/s390x/s390-hypercall.c
@@ -11,7 +11,7 @@
 
 #include "qemu/osdep.h"
 #include "cpu.h"
-#include "hw/boards.h"
+#include "hw/s390x/s390-virtio-ccw.h"
 #include "hw/s390x/s390-hypercall.h"
 #include "hw/s390x/ioinst.h"
 #include "hw/s390x/css.h"
@@ -57,6 +57,13 @@ static int handle_virtio_ccw_notify(uint64_t subch_id, uint64_t data)
     return 0;
 }
 
+static uint64_t handle_storage_limit(void)
+{
+    S390CcwMachineState *s390ms = S390_CCW_MACHINE(qdev_get_machine());
+
+    return s390_get_memory_limit(s390ms) - 1;
+}
+
 void handle_diag_500(S390CPU *cpu, uintptr_t ra)
 {
     CPUS390XState *env = &cpu->env;
@@ -69,6 +76,9 @@ void handle_diag_500(S390CPU *cpu, uintptr_t ra)
     case DIAG500_VIRTIO_CCW_NOTIFY:
         env->regs[2] = handle_virtio_ccw_notify(env->regs[2], env->regs[3]);
         break;
+    case DIAG500_STORAGE_LIMIT:
+        env->regs[2] = handle_storage_limit();
+        break;
     default:
         s390_program_interrupt(env, PGM_SPECIFICATION, ra);
     }
diff --git a/hw/s390x/s390-hypercall.h b/hw/s390x/s390-hypercall.h
index 2fa81dbfdd..4f07209128 100644
--- a/hw/s390x/s390-hypercall.h
+++ b/hw/s390x/s390-hypercall.h
@@ -18,6 +18,7 @@
 #define DIAG500_VIRTIO_RESET            1 /* legacy */
 #define DIAG500_VIRTIO_SET_STATUS       2 /* legacy */
 #define DIAG500_VIRTIO_CCW_NOTIFY       3 /* KVM_S390_VIRTIO_CCW_NOTIFY */
+#define DIAG500_STORAGE_LIMIT           4
 
 void handle_diag_500(S390CPU *cpu, uintptr_t ra);
 
-- 
2.47.1